RF transmitters are widely used in radio frequency communications systems, including but not limited to mobile radiotelephones. As is well known to those having skill in the art, an RF transmitter generally includes a modulator that modulates an input signal and a radio frequency power amplifier that is coupled to the modulator to amplify the modulated input signal. The radio frequency power amplifier is coupled to an antenna that transmits the amplified modulated input signal. The RF transmitter may be part of an RF transmitter/receiver (transceiver), wherein a duplexer or RF switch is placed between the RF power amplifier and the antenna, so that received communication signals from the antenna may be switched to a receive circuit.
RF transmitters are widely used in repeater-based communications systems, such as cellular and satellite mobile radiotelephone communications systems. Often, such systems allow a base station to send a command to the mobile radiotelephone to adjust its transmitter power. This allows a mobile radiotelephone to transmit at relatively high power when it is relatively far from a base station, and to transmit at relatively low power when the radiotelephone is relatively close to the base station. Accordingly, power control systems and methods can reduce battery drain in the mobile radiotelephone, to thereby allow longer standby time or transmission time. The mobile radiotelephone components may also have a longer operational life, due to reduced power use.
Reduced power transmission can also reduce interference between nearby channels of the same radiotelephone communications system or another system. The reduction of interference may be particularly important for cellular or satellite radiotelephone communications systems that use code division multiple access (CDMA) designs. More specifically, CDMA systems are susceptible to the "near-far" problem. The near-far problem occurs when one mobile radiotelephone is being received at a much higher power level than another mobile radiotelephone. For example, assume a second mobile radiotelephone is much closer to a base station than a first mobile radiotelephone. Thus, the second mobile radiotelephone will have a higher received power at the base station. Because there is little interference relative to the second mobile radiotelephone's received signal, it can be decoded properly. However, the signal from the first mobile radiotelephone may have a high bit error rate. Although the two signals may not be strongly correlated, the magnitude of the interference from the second mobile radiotelephone may be so strong that it degrades the first mobile radiotelephone signal.
The near-far problem is often reduced using power control systems and methods in mobile radiotelephones. Power control should preferably allow the transmitted signals from all mobile radiotelephones to be received at the same level and for the signals to be constant over time. Recent U.S. Patents that describe power control systems and methods include U.S. Pat. No. 5,551,057 to Mitra entitled "Cellular Mobile Radio System Power Control"; U.S. Pat. No. 5,606,285 to Wang et al. entitled "Power Control Circuit for Use With Transmitter"; U.S. Pat. No. 5,621,723 to Walton, Jr. et al. entitled "Power Control in a CDMA Network"; and U.S. Pat. No. 5,623,486 to Dohi et al. entitled "Transmission Power Control Method and Apparatus for Mobile Communications Using a CDMA (Code Division Multiple Access) System".
Power control systems and methods for transmitters of mobile radiotelephones generally operate over a limited power control range, such as 10 dB. This range has been obtained conventionally by providing a power control signal to the radio frequency power amplifier, to reduce the amplification thereof by up to 10 dB, in 1 dB steps. However, it would be desirable to obtain a wider range of power control. For example, in a mobile radiotelephone communications system, transmissions from a far radiotelephone (for example, 60 miles from a base station) may be received at the base station at a sensitivity level of -116 dBm. If a near radiotelephone (for example immediately adjacent the base station) is transmitting at a power of 30 dBm or 1 watt, then the effective power reaching the receiver may be about 0 dBm or 1 milliwatt, taking into account antenna coupling losses. If the base station has an Adjacent Channel Interference Power Ratio (ACIPR) of about 50 dB, then the preferred attenuation at the near radiotelephone is about 116 dB-50 dB, or about 66 dB. Accordingly, in order to reduce adjacent channel interference and the near-far problem, it would be desirable for a power control system to have a 60 dB range for a typical 1 watt mobile radiotelephone, compared to a conventional 10 dB range. With such a range, the desired distribution of power for mobile radiotelephones across the radius of the coverage area could be controlled as shown in FIG. 1.
Unfortunately, it may be difficult to obtain a wide range of power control in conventional RF transmitters. For example, in order to obtain a desired range of power control in a linear radio frequency power amplifier, the power amplifier may need to be operated in class-A. This may cause the amplifier to consume excessive DC power and may reduce the battery life considerably.
Other solutions to the near-far problem may reduce adjacent interference by providing narrower intermediate frequency (IF) filtering, to remove as much of the interfering signal as possible. This technique may be effective for some adjacent signals. However, as the adjacent signal falls within the desired bandwidth, filtering may be ineffective, because it also filters a desired signal along with the interference.
Another technique to reduce the near-far problem is to use multiple receivers and/or antennas and perform relatively complicated processing on the data to determine antenna/receiver correlation. The correlation information may then be used to subtract the interfering signal from the desired signal. Unfortunately, this technique may increase the cost and/or complexity of the mobile radiotelephone and/or base station.